How Much Nitrogen is Left in your Cornfield?

Source: University of Illinois

Thu, 2011-05-12 15:43

Wet soil conditions this spring are causing growers to question nitrogen (N) levels present in their soils and how that will affect this year's crop. Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition, says when soils become saturated, the potential for N loss is directly related to the amount of N present in the nitrate (NO–3-) form.

Under water-saturated conditions, nitrate is most likely to be lost through denitrification in fine-textured soils and through leaching below the root zone in coarse-textured soils or soils intensively tiled, Fernandez says. Most of the fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium.

Nitrification, or the conversion of ammonium to nitrate, is a bacteria-mediated transformation. The activity of these bacteria is minimal at temperatures below 50° F. These bacteria also need aerobic conditions (unsaturated soil-water conditions) to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil is largely dependent on soil temperature and the time elapsed from application until the soil becomes saturated with water.

"The nitrification process can be reduced with the use of nitrification inhibitors that will lower the activity of these bacteria and allow N to stay in the ammonium form for a longer period of time," he says.

The greatest reason for concern of N losses at this time would be if fall N application guidelines were not followed, Fernandez says.

"Last fall was warm well into the end of October and first part of November, with soil temperatures at the 4-in. depth dropping below 50° F later than normal," he says. "If fall application recommendations were not followed, there is a greater chance that some of the N might have been transformed to nitrate and potentially lost."

Another reason for concern is if growers used N sources not recommended for use in the fall, such as urea, ammonium nitrate or urea-ammonium nitrate solutions (UAN).

"Although temperatures were warm for a long time in the fall, once soil temperatures dropped below 50° to allow N applications, the temperatures remained low," he says. "I suspect that for N applied correctly last fall, there was very little chance for nitrification. I suspect that any nitrate that was present before soils froze was probably retained in the soil until the spring because much of the precipitation during the winter months did not move through the soil because soils were frozen."

Because soils have been cool and wet until recently, and nitrifying bacteria need warm temperatures and aerobic conditions to transform ammonium to nitrate, Fernandez says it is likely that little fertilizer N has been transformed to nitrate or soil-N mineralized at this point.